The present invention relates to an integrated air separation process in which a pre-purification unit is integrated with an air separation unit to remove impurities from the air prior to its separation. More particularly, the present invention relates to such a process in which the pre-purification unit contains adsorbent beds which operate in accordance with a pressure swing adsorption cycle. Even more particularly, the present invention relates to such a method in which the pressure swing adsorption cycle includes continuously subjecting the beds to a purge with a waste stream such as waste nitrogen.
Air is separated into its components by a variety of low temperature rectification processes. In accordance with such processes, air is compressed, cooled to a temperature suitable for its rectification, and then is introduced into an air separation unit which contains one or more columns in which the air is distilled into its component parts. The air separation unit can contain one or more distillation columns having trays or packing to provide intimate contact between vapor and liquid phases of the air to be separated.
In addition to oxygen, nitrogen and argon, the air also contains higher boiling components such as carbon dioxide, hydrocarbons and water. If these higher boiling components are not removed from the air prior to the cooling of the air, such components will condense during the cooling of the air and form ice in the main heat exchanger. Additionally, distillation columns tend to concentrate such higher boiling components as liquid column bottoms. In case of hydrocarbons, this can present an operational hazard. In order to prevent such problems, pre-purification units are provided that are integrated with the air separation plant in order to remove the higher boiling components such as moisture, carbon dioxide and hydrocarbons. Such pre-purification is effectuated by beds of adsorbent operating in accordance with pressure swing adsorption cycles.
A common pressure swing adsorption cycle has feed, depressurization, purge and re-pressurization stages. During the feed stage, compressed air is fed to a bed to produce a purified air stream which is then cooled in the main heat exchanger. After the feed stage is complete, the bed is depressurized to atmospheric pressure. The depressurization initiates adsorption of the adsorbed impurities from the adsorbent. Depressurization is completed by purging the bed with a waste stream, commonly waste nitrogen when available from a lower pressure column of a double column air separation unit. Thereafter, part of purified air from another bed undergoing a feed stage is routed to the bed that has just been subjected to a purge stage to repressurize the bed.
It is possible to use two beds which are operated out of phase such that while one bed is being fed, the other bed is regenerated and is therefore being subjected to depressurization, purge and repressurization stages. Even when more than two beds are used, the beds are only purged intermittently. As will be discussed, the inventor herein has found that operational efficiencies can be realized by operating a pressure swing adsorption cycle so that a waste stream is continuously being used to purge a bed undergoing the purge stage of the cycle.